High-precision fluid delivery systems such as mass flow controllers (MFCs) and mass flow ratio controllers (FRCs) are very important in applications such of semiconductor wafer fabrications. In many instances, the accuracy of these fluid delivery systems need to be verified.
A rate-of-rise (ROR) flow verifier may be used to verify the accuracy of measurement systems such as MFCs or FRCs. A typical ROR flow verifier may include a chamber, a pressure transducer, a temperature sensor and two isolation valves, one upstream and one downstream. The valves may be closed during idle, and may open when a run is initiated, allowing flow of fluid from the device under test (DUT) such as a MFC or a FRC through the flow verifier. Once fluid flow has stabilized, the downstream valve may be closed, and as a result the pressure may rise in the chamber, and the raise in pressure may be measured as well the gas temperature. These measurements may be used to calculate the flow rate and thereby verify the performance of the DUT.
The rising pressure in the chamber of a ROR verifier may be a major disturbance to the verification process. Although the DUT may adjust its valve position to offset the downstream pressure (chamber pressure) disturbance in order to maintain the targeted flow set point, the flow fluctuation may occur and undermine the flow rate verification process. A mass flow verification system and method are needed that can avoid such a disturbance to the DUT.
The connecting flow path volume between the DUT and the ROR flow verifier is called the external volume. It needs to be determined in order to calculate the flow rate by the ROR flow verifier. However, the setup calibration process for determining external volumes is very time-consuming if there are many DUTs connected to the ROR verifier so that a different external volume results for each DUT. Furthermore, the accuracy of flow verification by a ROR decreases as the external volume increases. This is because the pressure drop along the flow path, i.e. the pressure change (measured by the pressure transducer) in the chamber of a ROR, is different from the pressure change along the flow path. The longer the flow path, the lesser the accuracy of flow verification. A mass flow verification system and method are needed in order to solve the external volume problem for the ROR verifier.